Why Do We Have Moral Obligations?

If God does not exist, why think that we have any moral obligations to do anything? Who or what imposes these moral duties upon us? Where did they come from? It's very hard to see why they would be anything more than a subjective impression ingrained into us by societal and parental conditioning. If there is no moral law giver, then there is no objective moral law which we must obey. Given the finality of death, it really does not matter how you live.